On validity of the quasi-static approximation in scalar-tensor theories

Understand

The discovery of cosmic acceleration motivated extensive studies of dynamical dark energy and modified gravity models.

• Of particular interest are the scalar-tensor theories, with a scalar field dark energy non-minimally coupled to matter.
• Cosmological constraints on these models often employ the quasi-static approximation (QSA), in which the dynamics of the scalar field perturbations is proportional to the perturbation in the matter density.
• Using the QSA simplifies the physical interpretation of the phenomenology of scalar-tensor theories, and results in substantial savings of computing time when deriving parameter constraints.